This invention relates to the use of normally wet quaternary ammonium compounds as accelerators for the polymerization of 2-pyrrolidone. The anionic polymerization of this substance is well known in the art and was first disclosed in U.S. Pat. No. 2,638,463. Improved methods of polymerizing 2-pyrrolidone are disclosed in many subsequent patents, for example U.S. Pat. No. 3,721,652, but all involve the use of an alkali metal derivative of 2-pyrrolidone, or similar derivative, as a catalyst. To initiate the polymerization an additional substance customarily referred to as an "activator" is required. Preferred activators are CO.sub.2 and SO.sub.2 although many other substances such as N-acetyl pyrrolidone may be used. In order for polymerization to occur the system must be substantially anhydrous. It has been determined that the presence of more than about 0.1% of water, based on the 2-pyrrolidone monomer, effectively retards or prevents polymerization and less than this amount of water is preferred.
As disclosed in out co-pending application Ser. No. 39,773, now U.S. Pat. No. 4,217,442, the addition of certain quaternary ammonium compounds, especially the sulfates and bisulfates, causes a marked increase in the rate of polymerization of 2-pyrrolidone. Such an increase in the polymerization rate is of great commercial value. But in order to achieve this and indeed in order to effect any polymerization at all it is essential that the system be anhydrous. Obviously the quaternary ammonium compound cannot simply be added to the polymerization mixture since all quaternary ammonium compounds are very hygroscopic and therefore normally wet. They are also thermally unstable and drying by heating even under vacuum is likely to result in some decomposition. The operation is at best a compromise between the need to raise the temperature to a point where all the water may be removed and the need to keep the temperature below the point where thermal decomposition may occur. When intended for addition to a 2-pyrrolidone polymerization mixture, even slight thermal decomposition is almost as detrimental as having some water present since the amines formed in the decomposition are inhibitors to the polymerization.
It is therefore the object of this invention to provide a method of polymerizing 2-pyrrolidone in which the accelerating effect of the quaternary ammonium compound may be utilized.